1. Field of the Invention
The present invention relates to an improvement in a method of cleaning semiconductor wafers in a strong-alkaline aqueous solution at a point of time after lapping and before etching in the manufacture of semiconductor wafers of silicon, GaAs, GaP, InP, or the like.
2. Description of the Related Art
Generally, the manufacture of semiconductor mirror-polished wafers is composed of the steps of slicing a monocrystalline ingot of silicon or the like into semiconductor wafers; and chamfering, lapping, acid-etching, and then mirror-polishing the thus-obtained semiconductor wafers.
In the manufacture of semiconductor mirror-polished wafers, various substances adhere to semiconductor wafers which have undergone lapping. Such adhering substances include heavy metal; lapping powder composed of aluminum oxide, silicon carbide, and the like; and a rust preventive contained generally in a lapping solution. In order to remove such adhering substances, lapped semiconductor wafers undergo cleaning before entering the subsequent acid-etching step. According to a conventionally employed cleaning method, before the acid-etching step, lapped semiconductor wafers are immersed in a strong-alkaline aqueous solution showing strong corrosion on wafers so as to slightly dissolve the surfaces of the wafers (amount of dissolution: approximately 1 .mu.m to 2 .mu.m) and to simultaneously remove adhering substances such as the lapping powder and rust preventive.
However, when a semiconductor wafer which has been cleaned in a strong-alkaline aqueous solution is acid-etched, as shown in FIG. 1, a protrusion 2 having a height of up to approximately 25 .mu.m is generated on the outer circumferential end portion 1 of the semiconductor wafer.
Before a semiconductor wafer undergoes lapping, the outer circumferential end portion thereof undergoes chamfering. The chamfered outer circumferential end portion suffers excessive mechanical damage (depth: approximately 5 .mu.m to 15 .mu.m) induced by chamfering. As a result, when the semiconductor wafer is cleaned in a strong-alkaline aqueous solution, the outer circumferential end portion is selectively dissolved, resulting in the formation of a number of pits. Consequently, the surface of the outer circumferential end portion becomes rough.
When a semiconductor wafer having the thus-roughened outer circumferential end portion is sent to the subsequent acid-etching step, the roughened outer circumferential end portion scrapes a basket for conveying or cleaning semiconductor wafers. As a result, thus-produced scrapings of the basket adhere to the pits and serve as masks that thereby prevent etching of the corresponding portions, resulting in the generation of a protrusion.
Further, foreign matter trapped in the pits remains as contaminants even after the completion of acid-etching.